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Difference between revisions of "EclipseLink/Examples/MOXy/JPA"

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==== Many To Many ====
 
==== Many To Many ====
  
In JPA the ManyToMany annotation indicates that one or more instances of the source entity are able to refer to the same target entity instance.
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In JPA the ManyToMany annotation indicates that one or more instances of the source entity are able to refer to the same target entity instance. Since the relationship between Department and Employee is bi-directional, the EclipseLink extension @XmlInverseReference can be used to represent the back-pointer
  
 
<source lang="java">
 
<source lang="java">

Revision as of 15:21, 27 November 2009

Overview

This example demonstrates how to derive an XML representation from a set of JPA entities using JAXB. In simplest terms JAXB will be used to apply a tree structure to the graph of JPA entities. Multiple tree representations of a graph are possible and will depend on the root object chosen and the direction the relationships are traversed.

JPA Entities

For this example the following JPA entities will be used. In order to save space package names, import statements, and the get /set methods have been omitted from the following code samples. All annotations shown below are standard JPA annotations.

@Entity
public class Department {
 
    @Id
    @Column(name="D_ID")
    private BigDecimal dId;
 
    private String name;
 
    @ManyToMany
    @JoinTable(name="DEPT_EMP", joinColumns = 
        @JoinColumn(name="D_ID", referencedColumnName = "D_ID"), 
            inverseJoinColumns = @JoinColumn(name="E_ID", 
                referencedColumnName = "E_ID"))
    private List<Employee> member;
 
}
@Entity
public class Employee {
 
    @Id
    @Column(name="E_ID")
    private BigDecimal eId;
 
    private String name;
 
    @OneToOne(mappedBy="resident")
    private Address residence;
 
    @OneToMany(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
    @ManyToMany(mappedBy="member")
    private List<Department> team;
 
}
@Entity
public class Address {
 
    @Id
    @Column(name="E_ID", insertable=false, updatable=false)
    private BigDecimal eId;
 
    private String city;
 
    private String street;
 
    @OneToOne
    @JoinColumn(name="E_ID")
    private Employee resident;
 
}
@Entity
@Table(name="PHONE_NUMBER")
public class PhoneNumber {
 
    @Id
    @Column(name="P_ID")
    private BigDecimal pId;
 
    @ManyToOne
    @JoinColumn(name="E_ID", referencedColumnName = "E_ID")
    private Employee contact;
 
    private String num;
 
}

JAXB Binding

For this example the XML acessor type will be set to FIELD for all the model classes. This can be set as a package level JAXB annotation.

@XmlAccessorType(XmlAccessType.FIELD)
package com.example.model;
 
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;

Relationships - Privately Owned

These relationships apply when the target object(s) is only referenced by a single source object. This type of relationship can be safely represented as nesting in XML (the default in JAXB).

One To One and Embedded

In JPA the OneToOne and Embedded annotations indicate that only one instance of the source entity is able to refer to the same target entity instance. Since the relationship between Employee and Address is bi-directional, the EclipseLink extension @XmlInverseReference can be used to represent the back-pointer.

@Entity
public class Employee {
 
    @OneToOne(mappedBy="resident")
    private Address residence;
 
}
@Entity
public class Address {
 
    @OneToOne
    @JoinColumn(name="E_ID")
    @XmlInverseReference(mappedBy="residence")
    private Employee resident;
 
}

One To Many

In JPA the OneToMany annotation indicates that only one instance of the source entity is able to refer to the same target entity instance. Since the relationship between Employee and Address is bi-directional, we can again use the EclipseLink extension @XmlInverseReference to map the back-pointer.

@Entity
public class Employee {
 
    @OneToMany(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
}
@Entity
public class PhoneNumber {
 
    @ManyToOne
    @JoinColumn(name="E_ID", referencedColumnName = "E_ID")
    @XmlInverseReference(mappedBy="contactNumber")
    private Employee contact;
 
}

Relationships - Shared Reference

These relationships apply when the target object(s) may be referenced by multiple source objects. This type of relationship can not be safely represented as nesting in XML. So instead key relationships are used. In order to leverage the ID fields on JPA entities, EclipseLink JAXB allows the @XmlID annotation to be used on non-String fields/properties.

Many To One

In JPA the ManyToOne annotation indicates that one or more instances of the source entity are able to refer to the same target entity instance. Since the relationship between PhoneNumber and Employee is bi-directional, the EclipseLink extension @XmlInverseReference can be used to represent the back-pointer

@Entity
public class PhoneNumber {
 
    @ManyToOne
    @JoinColumn(name="E_ID", referencedColumnName = "E_ID")
    @XmlIDREF
    private Employee contact;
 
}
@Entity
public class Employee {
 
    @Id
    @Column(name="E_ID")
    @XmlID
    private BigDecimal eId;
 
    @OneToMany(mappedBy="contact")
    @XmlInverseReference(mappedBy="contact")
    private List<PhoneNumber> contactNumber;
 
}

Many To Many

In JPA the ManyToMany annotation indicates that one or more instances of the source entity are able to refer to the same target entity instance. Since the relationship between Department and Employee is bi-directional, the EclipseLink extension @XmlInverseReference can be used to represent the back-pointer

@Entity
public class Department {
 
    @ManyToMany
    @JoinTable(name="DEPT_EMP", joinColumns = 
        @JoinColumn(name="D_ID", referencedColumnName = "D_ID"), 
            inverseJoinColumns = @JoinColumn(name="E_ID", 
                referencedColumnName = "E_ID"))
    @XmlIDREF
    private List<Employee> member;
 
}
@Entity
public class Employee {
 
    @Id
    @Column(name="E_ID")
    @XmlId
    private BigDecimal eId;
 
    @ManyToMany(mappedBy="member")
    @XmlInverseReference(mappedBy="member")
    private List<Department> team;
 
}